1. Field of the Invention
The present invention relates to methods for correction in lithographic projection apparatus, and more particularly to flare correction in lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is extremely important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer; using optical and electronic components in combination with substrate holder positioning actuators (referred to hereinafter as “alignment system”), this mark can then be re-located each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
There is a need to produce smaller and smaller semiconductor devices, and thus a corresponding need to provide projection systems enabling projection of features with smaller critical dimension (referred to hereinafter as “CD”). Thus, these lithography apparatus are being pushed to their resolution limits, while maintaining sufficient “process latitude” (i.e., sufficient depth of focus and sufficient insensitivity to residual errors in the dose of exposure of irradiated target portions). Therefore, there is a need to minimize factors which can affect the resolution of the apparatus and the process latitude, and consequently there is a need to provide accurate monitoring for these factors.
Many different factors can affect the smallest CD and the process latitude for a given lithographic projection apparatus, such as, for example, residual aberrations of the projection system, focus and dose errors, and the occurrence of stray radiation. In particular, the more flare due to stray radiation that is present in the image of a pattern projected by the projection system, the less the resolution that can be obtained, and the smaller the process latitude will be. Stray radiation may, for example, be caused by scattering of projection beam radiation at contaminating particles and/or defects on surfaces of optical elements of the projection system. Also, optical elements provided with anti-reflection coatings may cause stray radiation due to degradation of materials used for the anti-reflection coatings. Degradation of materials may be a radiation induced effect, and like the number of contaminating particles and/or defects it may increase as a function of time.
CD uniformity (CDU) is a critical imaging parameter for imaging of a single pattern feature. However, if a plurality of pattern features are to be reproduced simultaneously, not only is the individual CDU for each feature of importance, but also the average CD for each feature which should be in the desired target CD range. Any mismatch in the target CD of a feature constitutes an additional contribution to the overall CDU (that is the total CDU for all features). It is therefore important that the reproduction of each pattern feature should be on target as far as possible. This is particularly important when a mask optical proximity correction (OPC) is applied to obtain a specific imaging performance. In this case a single pattern feature (for instance a series of dense horizontal lines) can be printed on target by applying a dose correction. In order to optimize CDU for multiple critical features (for instance simultaneous optimization of dense and isolated lines) additional corrections may be applied to the mask, often referred to as mask OPC. In general the mask OPC is determined once for each mask design and then is not changed. However one of the factors that contributes to the offset with respect to target CD for multiple features is the stray radiation that generally varies over time. The printing performance for multiple features of an exposure tool may therefore be off target as a result of variation in the stray radiation levels even when a mask with OPC is used.